Graphene is a substantially two-dimension equivalent nano material formed in a sheet shape of carbon atoms. The material exhibits extremely superior properties in high current density resistance, superhigh mobility, high heat resistance, and high mechanical strength. Therefore, like a carbon nanotube, graphene is regarded as a promising wire material of semiconductor devices. For example, it is theoretically predicted that a graphene nanoribbon whose width is processed up to about 10 nm exhibits electric conductivity exceeding that of copper. To actually produce a wire structure by using graphene as a wire material, it is necessary to combine with materials of electrodes, dielectric films and the like. In contrast to conventional wire materials such as aluminum (Al), tungsten (W), and copper (Cu), graphene is a sheet of layer of several atoms and thus, characteristics (carrier density, carrier type, mobility and the like) thereof change significantly depending on a material in contact therewith. For example, the Dirac point drops by about −0.6 eV in a portion where Al as an electrode is in contact with a crosslinking portion up in the air with only a graphene film due to a doping effect of Al. The crosslinking portion is thereby doped with n-type. As a result, the carrier density in the Al contact portion increases, but does not change in the crosslinking portion and thus, an npn junction is formed between −0.6 V to 0 V of the applied voltage. Therefore, there is a problem of the formation of a low conductivity region.
It is normally difficult to produce a crosslinking portion with a wire structure in a semiconductor device. Thus, a structure in which an upper-layer wire and a lower-layer wire are insulated by a dielectric film is generally adopted. However, like Al described above, the Dirac point of graphene is shifted by several +eV by being brought into contact with a dielectric film, for example, a silicon oxide film. Regions other than a region of contact come into contact with a dielectric film in the wire structure and thus, a still wider low conductivity region than when Al is brought into contact as an electrode will be formed. This means that the wire structure is ill-suited and a graphene wire structure that reduces a low conductivity region to a minimum is needed.